Wait a tick - Davy Jones is apart of the story and his heart is the thing that is important, the East India Trading Co took it to take over the seas (because Davy Jones rules the sea) and thus threatening the very existence of piracy. And with that note, it makes sense [blackout]why the kraken is just on the beach in the third one to emphasize that Davy is being controlled and the EITC doesn't want him betraying them so they killed the kraken[/blackout].
The back story I assume you're talking about is [blackout]the romance between him and Calypso. This was apart of the pirate's defence (headlined by Barbossa, who knew of Tia Dalma's secret) and Davy was also apart of the EITC finding out about Calypso. It was just "that time of the month" for Calypso, she goes full out angry over everyone because of her being trapped in the human form[/blackout].
Everything is in the movies for a reason. It all makes sense to me...